1. Field of the Invention
The present invention relates to an air conditioning system and a vehicular air conditioning system using a refrigerant comprising carbon dioxide, and relates to a control method of the vehicular air conditioning system.
2. Description of Related Art
In an air conditioning system using a refrigerant comprising carbon dioxide (CO2), since there is a high pressure refrigerant pressure value which is efficient with respect to a temperature of a high pressure refrigerant on the outlet side of a radiator, the temperature of the high pressure refrigerant on the outlet side of the radiator is detected to calculate a target refrigerant pressure, a pressure reducing unit such as an expansion valve is controlled based on the target refrigerant pressure bring the refrigerant pressure close to the target pressure. In the air conditioning system using a refrigerant comprising carbon dioxide, since the refrigerant pressure is high, if the number of joints is increased, the refrigerant tends to leak. Therefore, the refrigerant temperature is not measured directly, and the temperature of a pipe surface through which the refrigerant passes is measured. When the air conditioning system is applied to a vehicular air conditioning system, since it is necessary to increase the refrigerant pressure at an outlet of a radiator, an engine load is increased. Based on such a background, as disclosed in Japanese Patent No. 2931668, in the vehicular air conditioning system to which the air conditioning system is applied, opening of the expansion valve is adjusted such that the refrigerant pressure at the outlet of the radiator has a predetermined relation with respect to the refrigerant temperature at the outlet, thereby obtaining the maximum efficiency.